Rapid measurement of sphingolipids from Arabidopsis thaliana by reversed-phase high-performance liquid chromatography coupled to electrospray ionization tandem mass spectrometry

Changes in sphingolipids have been associated with profound effects in cell fate and development in both plants and animals. Sphingolipids as a group consist of a large number of different compound classes of which numerous individual species may vary in response to environmental stimuli to affect cellular responses. The ability to measure all sphingolipids simultaneously is, therefore, essential to an understanding of the biochemical regulation of sphingolipid metabolism and signaling molecules derived from it. In the model plant Arabidopsis thaliana, the major sphingolipid classes are glycosylinositolphosphoceramides, glucosylceramides, hydroxyceramides and ceramides. Other minor but potentially important sphingolipids are free long-chain bases and their phosphorylated derivates. By using a single solvent system with reversed-phase high-performance liquid chromatography coupled to electrospray ionization tandem mass spectrometry detection we have been able to separate and measure 168 sphingolipids from a crude sample. This greatly speeds up and simplifies the analysis of plant sphingolipids and should pave the way for a better understanding of their role in plant performance.     

1H NMR direct observation of enantiomeric exchange in palladium(II) and platinum(II) complexes containing N,N' bidentate aryl-pyridin-2-ylmethyl-amine ligands

The complexes [MCl(2)(kappa2-N approximately N')] (N approximately N' = 2-C(5)H(4)N-CH2-NHAr; Ar = 4-MeC(6)H(4), a; 2,6-Me(2)C(6)H(3), b; 4-MeOC(6)H(4), c; 4-CF(3)C(6)H(4), d; M = Pd, 1a-d; Pt, 2a-d) have been prepared and fully stereochemically characterized both in the solid state and in solution. Their behavior in DMSO-d6 solution is dependent on the substituents of the aryl group and on the metal. Complexes of palladium with substituents at the para position (1a, 1c, 1d) display a dynamic 1H NMR pattern when the solutions are heated. An enantiomeric exchange Slambda/Rdelta is suggested to explain such behavior. On the basis of the calculated negative DeltaS values, an associative mechanism involving the solvent is proposed. Under the same conditions, analogous complexes of platinum (2a, 2c, 2d) proved to be unstable, and release of the N approximately N' ligand was observed. Complexes 1b and 2b show temperature-variable 1H NMR spectra without any evidence accounting for enantiomeric exchange or decoordination. DFT calculations on models of 1a and 1b show that diastereomeric exchange Sdelta/Slambda is a process where the complex with the higher sterical hindrance, 1b, has a lower energy barrier.     

Redox control of meso-zinc(II) ferrocenylporphyrin based fluorescence switches

The efficient on/off switching of fluorescence from thienyl- and phenyl-substituted porphyrin-ferrocene dyads is achieved through redox control of excited-state electron-transfer quenching.     

Infrared and DFT investigations of the XC[triple bond]ReX3 and HC[triple bond]ReX3 complexes: Jahn-Teller distortion and the methylidyne C-X(H) stretching absorptions

The XC[triple bond]ReX3 complexes (X = F, Cl) are produced by CX(4) reaction with laser-ablated Re atoms, following oxidative C-X insertion and alpha-halogen migration in favor of the carbon-metal triple bond and are identified through the observation of characteristic absorptions in the argon matrix infrared spectra and comparison with vibrational frequencies calculated by density functional theory. The methylidyne C-F and C-Cl stretching absorptions are observed near 1584 and 1328 cm-1, and the C-H stretching modes for HC[triple bond]ReX3 at 3104 and 3097 cm(-1), respectively, which are substantially higher than the precursor stretching modes and in agreement with the general trend that higher s-orbital character in carbon hybridization leads to a higher stretching frequency. The Jahn-Teller effect in the doublet-state XC[triple bond]ReX3 and HC[triple bond]ReX3 complexes gives rise to distorted structures with Cs symmetry and two equivalent longer Re-X bonds and one slightly shorter Re-X bond.     

Reactions of CF3CH2I+O(3P): competing mechanisms of HF elimination

Ab initio density functional and molecular orbital calculations provide singlet and triplet electronic potential energy surfaces for the reactions of CF3CH2I+O(3P) leading to OI and HF eliminations, reactions which have been the subject of recent experimental studies. A barrier to OI formation occurs on the triplet potential energy surface; there is no reverse barrier to OI formation on the singlet pathway. Findings suggest that two competing pathways may form HF. One is an addition-insertion-elimination process involving insertion of O into the C-I bond. The alternate path involves OI elimination, addition of an O atom to CF3CH2, and subsequent HF elimination. The computed reactant pathways and energetics are discussed in relation to recent experiments.     

Infrared spectrum and bonding in uranium methylidene dihydride, CH2=UH2

Uranium atoms activate methane upon ultraviolet excitation to form the methyl uranium hydride CH3-UH, which undergoes alpha-H transfer to produce uranium methylidene dihydride, CH2=UH2. This rearrangement most likely occurs on an excited-quintet potential-energy surface and is followed by relaxation in the argon matrix. These simple U+CH4 reaction products are identified through isotopic substitution (13CH4, CD4, CH2D2) and density functional theory frequency and structure calculations for the strong U-H stretching modes. Relativistic multiconfiguration (CASSCF/CASPT2) calculations substantiate the agostic distorted C1 ground-state structure for the triplet CH2=UH2 molecule. We find that uranium atoms are less reactive in methane activation than thorium atoms. Our calculations show that the CH2=UH2 complex is distorted more than CH2=ThH2. A favorable interaction between the low energy open-shell U(5f) sigma orbital and the agostic hydrogen contributes to the distortion in the uranium methylidene complexes.     

The radical character of the acenes: a density matrix renormalization group study

We present a detailed investigation of the acene series using high-level wave function theory. Our ab initio density matrix renormalization group algorithm has enabled us to carry out complete active space calculations on the acenes from napthalene to dodecacene correlating the full pi-valence space. While we find that the ground state is a singlet for all chain lengths, examination of several measures of radical character, including the natural orbitals, effective number of unpaired electrons, and various correlation functions, suggests that the longer acene ground states are polyradical in nature.     

Preferential attachment during the evolution of a potential energy landscape

It has previously been shown that the network of connected minima on a potential energy landscape is scale-free, and that this reflects a power-law distribution for the areas of the basins of attraction surrounding the minima. Here, the aim is to understand more about the physical origins of these puzzling properties by examining how the potential energy landscape of a 13-atom cluster evolves with the range of the potential. In particular, on decreasing the range of the potential the number of stationary points increases and thus the landscape becomes rougher and the network gets larger. Thus, the evolution of the potential energy landscape can be followed from one with just a single minimum to a complex landscape with many minima and a scale-free pattern of connections. It is found that during this growth process, new edges in the network of connected minima preferentially attach to more highly connected minima, thus leading to the scale-free character. Furthermore, minima that appear when the range of the potential is shorter and the network is larger have smaller basins of attraction. As there are many of these smaller basins because the network grows exponentially, the observed growth process thus also gives rise to a power-law distribution for the hyperareas of the basins.     

Mechanistic studies on the Heck-Mizoroki cross-coupling reaction of a hindered vinylboronate ester as a key approach to developing a highly stereoselective synthesis of a C1-C7 Z,Z,E-triene synthon for viridenomycin

Mechanistic studies of the Heck-Mizoroki reaction of a vinylboronate ester with electronically different (four-substituted) aryl iodides shows that electron donors accelerate the cross-coupling, demonstrating that the oxidative addition step is not rate determining and that there is development of some degree of positive charge in the rate determining step. These results were used as a basis to allow the development of reaction conditions for the Heck-Mizoroki coupling of a hindered vinylboronate ester with electron deficient methyl cis-2-iodoacrylate. The resulting dienylboronate ester was converted through a series of highly stereoselective iodo-deboronations and Heck-Mizoroki reactions into a trienyl iodide precursor for further application in the total synthesis of viridenomycin.     

Core-modified, meso-alkylidenyl porphyrins and their expanded analogues: a new family of nonplanar porphyrinoids

New core-modified, meso-alkylidenyl porphyrinoids bearing multiple exocyclic double bonds were synthesized and characterized. The synthesis was accomplished using a typical "3 + 1"-type condensation approach. Stable exocyclic tautomers bearing double bonds at the meso positions, as well the corresponding endocyclic tautomers, were isolated in the case of both thiabenziporphyrin and thiapyriporphyrin products prepared in the course of this study. On the other hand, only the exocyclic tautomer was isolated in the case of the congeneric oxapyriporphyrin and oxabenziporphyrin. Expanded analogues of the exocyclic forms of oxabenziporphyrin and thiabenziporphyrin were also isolated as minor products. A single-crystal X-ray diffraction analysis of the expanded thiabenziporphyrin (20) revealed that all four pyrrole rings displayed an inverted geometry, presumably reflecting the strong hydrogen-bonding extant between the pyrrole N-H proton and the carbonyl group of the malonate moiety in the solid state. On the other hand, the expanded oxabenziporphyrin (14) was found to possess a severely distorted geometry with only one pyrrole ring being inverted. Careful analysis of the structure revealed that the solid-state geometry of the expanded macrocycles correlates well with the internal angle defined by the 2- and 5 substituents and the centers of the furan (14) or thiophene (20) subunits.